The present invention relates to fuel cells and, more particularly, to ceramic honeycomb fuel cells including an oxygen ion conducting ceramic interposed between an oxidant supply cathode electrode and a fuel supply anode electrode.
Solid electrolyte fuel cells include a solid electrolyte that is oxygen-ion conductive. A porous cathode electrode and a porous anode electrode are formed on opposite sides of the electrolyte. An oxidant, e.g., oxygen gas or air, is introduced into an oxidant supply passage on the cathode side of the electrolyte. A fuel, e.g., hydrogen gas or natural gas, is introduced into a fuel supply passage on the anode side of the electrolyte. Oxygen molecules in the oxidant supply passage dissociate at the cathode electrode and absorb electrons to form oxygen ions. These ions then diffuse through the ionic conductor to the anode electrode, leaving the cathode entry surface with a deficiency of electrons. Oxygen ions leaving the anode electrode must give up electrons to form molecular oxygen, thus leaving the anode exit surface with an excess of electrons. In this manner, the fuel cell utilizes the oxygen ion conductivity of the electrolyte to function as an electrical current source.
Many fuel cells must be operated at temperatures above 800xc2x0 C. and as high as 1000xc2x0 C. Natural gas and methane tend to cause sooting within the fuel supply passages at these elevated temperatures. As a result, it is often necessary to reform the natural gas into a substantially pure hydrogen gas prior to introducing it into the fuel supply passages. Accordingly, there is a need for a fuel cell that is not susceptible to sooting and does not require reformation of a natural gas supply.
There is also a continuing drive to decrease production costs and increase efficiency of the above-described fuel cells through optimal selection of cathode electrode, anode electrode, and electrolyte materials or arrangements. For example, U.S. Pat. No. 5,807,642 (Xue et al.) teaches a barium strontium titanate ceramic body including material additives that serve as modifiers of the coefficient of thermal expansion or as sintering processing aids. U.S. Pat. No. 5,731,097 (Miyashita et al.) relates to a solid-electrolyte fuel cell including first and second oxygen ion conductive films stuck together and arranged in descending order, toward the anode, by oxygen ion activation energy. U.S. Pat. No. 5,712,055 (Khandkar et al.) teaches a multi-stage arrangement for the electrolyte material in a fuel cell. The disclosures of each of these patent references are incorporated herein by reference. Although each of the above-mentioned schemes, like other conventional fuel cell schemes, purport to present an optimal fuel cell arrangement, there still exists a need in the art for an improved fuel cell arrangement.
This need is met by the present invention wherein a ceramic fuel cell is provided including, among other things, (i) an yttria stabilized bismuth oxide oxygen ion conductive ceramic including ZrO2, (ii) a niobia stabilized bismuth oxide oxygen ion conductive ceramic, (iii) a copper cermet anode electrode disposed in the fuel supply passage of a bismuth oxide ceramic fuel cell, or (iv) specially arranged inter-passage channels formed in the ceramic body of the fuel cell.
In accordance with one embodiment of the present invention, a ceramic fuel cell is provided comprising an oxidant supply passage, a cathode electrode disposed in the oxidant supply passage, a fuel supply passage, an anode electrode disposed in the fuel supply passage, an yttria stabilized bismuth oxide oxygen ion conductive ceramic interposed between the cathode electrode and the anode electrode. The ceramic includes ZrO2. A zirconia coating may be interposed between the yttria stabilized ceramic and the anode electrode. The yttria stabilized ceramic preferably comprises x mole % Bi2O3, y mole % Y2O3, and z mole % ZrO2, wherein x is a value from about 70 to about 80, y is a value from about 20 to about 30, and z is a value from about 1 to about 5.
In accordance with another embodiment of the present invention, a ceramic fuel cell is provided comprising an oxidant supply passage, a cathode electrode disposed in the oxidant supply passage, a fuel supply passage, an anode electrode disposed in the fuel supply passage, and a niobia stabilized bismuth oxide oxygen ion conductive ceramic interposed between the cathode electrode and the anode electrode. The niobia stabilized ceramic preferably comprises x mole % Bi2O3 and y mole % Nb2O5, wherein x is a value from about 80 to about 90, and wherein y is a value from about 10 to about 20.
Preferably, either the cathode electrode, the anode electrode, or both comprise a ceramic electrode. The ceramic electrode material may be characterized by the ceramic composition LXM, where L is lanthanum (La), M is manganate (MnO3), and X is lead (Pb). A silver layer may be disposed over the ceramic electrode material and may comprise a glass mixed therein, wherein the glass is selected so as to enhance adhesion of the silver layer to the ceramic electrode material.
In some embodiments of the present invention, the anode electrode comprises a copper cermet. The copper cermet may comprise a mixture of powders of CuO and a bismuth oxide ceramic. The bismuth oxide ceramic may comprise a niobia stabilized bismuth oxide oxygen ion conductive ceramic.
The oxygen ion conductive ceramic may be arranged to define a plurality of oxidant supply passages and a plurality of fuel supply passages. The oxidant supply passages may be oriented substantially parallel to the fuel supply passages and selected ones of the oxidant supply passages are preferably defined so as to be adjacent to corresponding ones of the fuel supply passages. More specifically, the oxygen ion conductive ceramic may be arranged to define a plurality of substantially parallel longitudinal channels and selected ones of the longitudinal channels may define the oxidant supply passages and remaining ones of the longitudinal channels define the fuel supply passages.
Further, the oxygen ion conductive ceramic body defining the oxidant supply passage and the fuel supply passage may be in the form of first and second sets of substantially parallel passages, wherein (i) each of the passages defines opposite passage ends, (ii) the opposite ends of the first set of passages are open, (iii) the opposite ends of the second set of passages are closed, (iv) the second set of passages include inter-passage channels formed in the ceramic body between adjacent ones of the second set of passages, and (v) the inter-passage channels are arranged proximate selected ones of the opposite passage ends. An input port and an output port may be coupled to the second set of passages, wherein the second set of passages, the input port, the output port, and the inter-passage channels are arranged to define a flow path extending from the input port, through the second set of passages and the inter-passage channels, to the output port. The inter-passage channels are preferably defined at opposite end faces of the ceramic body.
In accordance with yet another embodiment of the present invention, a ceramic fuel cell is provided comprising an oxidant supply passage, a cathode electrode disposed in the oxidant supply passage, a fuel supply passage, a copper cermet anode electrode disposed in the fuel supply passage, and a bismuth oxide oxygen ion conductive ceramic interposed between the cathode electrode and the anode electrode. The copper cermet anode electrode preferably comprises a mixture of powders of CuO and a bismuth oxide ceramic.
In accordance with yet another embodiment of the present invention, a ceramic fuel cell is provided comprising an oxygen ion conductive ceramic body defining first and second sets of substantially parallel passages, wherein (i) each of the passages define opposite passage ends, (ii) the opposite ends of the first set of passages are open, (iii) the opposite ends of the second set of passages are closed, (iv) the second set of passages include inter-passage channels formed in the ceramic body between adjacent ones of the second set of passages, and (v) the inter-passage channels are arranged proximate selected ones of the opposite passage ends. Respective first electrodes are disposed in the first set of passages and respective second electrodes are disposed in the second set of passages. An input port and an output port are coupled to the second set of passages. The second set of passages, the input port, the output port, and the inter-passage channels are arranged to define a flow path extending from the input port, through the second set of passages and the interpassage channels, to the output port. Preferably, the input port is coupled to a fuel supply and the first set of passages are coupled to an oxidant supply such that the respective first electrodes comprise cathode electrodes and the respective second electrodes comprise anode electrodes. Alternatively, the input port may be coupled to an oxidant supply and the first set of passages may be coupled to a fuel supply such that the respective first electrodes comprise anode electrodes and the respective second electrodes comprise cathode electrodes.
The fuel cell may further comprise a manifold assembly defining: (i) an input manifold coupled to a first end face of the ceramic body, wherein the input manifold defines a first manifold input in communication with the first set of passages; (ii) an output manifold coupled to an opposite end face of the ceramic body, wherein the output manifold defines a first manifold output in communication with the first set of passages; (iii) and a side face manifold coupled to opposite side faces of the ceramic body, wherein the side face manifold defines a second manifold input in communication with the input port and a second manifold output in communication with the output port. The side face manifold and the output manifold may comprise a unitary manifold assembly.
Accordingly, it is an object of the present invention to provide a ceramic fuel cell that is less expensive to produce and that embodies improved operating characteristics. Other objects of the present invention will be apparent in light of the description of the invention embodied herein.